levels. >> reporter: relief for those who received the j&j vaccine. keith alberts admits he's been concerned. >> i feel a whole lot better. i feel like this is what i've been waiting for. i've been waiting to hear this. there was nothing ever mentioned about a booster for johnson. it was all pfizer and moderna. >> reporter: with the delta variant spreading fast, the biden administration is eager to offer boosters, six to eight months after americans are fully vaccinated starting next month. but is it safe to mix boosters? giving a pfizer or moderna booster to someone who got the j&j shot? overseas study suggest yes. >> the data suggests mixing johnson & johnson and say pfizer or moderna would lead to incredibly high effectiveness levels and that's very promising . >> it's not clear how long the protection would last. the boost comes as more government agencies and